S4lt macros

• s4_atrim — trims leading and trailing whitespace from all arguments.
• s4_include — includes file.
• s4_ldrop — drops empty leading arguments.
• s4_left — returns characters from the left side of the string.
• s4_ltrim — trims leading whitespace.
• s4_rdrop — drops empty trailing arguments.
• s4_right — returns characters from the right side of the string.
• s4_rtrim — trims trailing whitespace.
• s4_trim — trims leading and trailing whitespace.
• s4_x — repeats the string.